Silicone resins have been used for a number of purposes because of their relative thermal stability, dielectric properties, chemical stability and resistance to atmospheric deterioration. The U.S. Pat. No. 4,564,562 of C. P. Wong is an example of the prior art showing the use of silicone resin elastomers as encapsulants for electronic devices, particularly hybrid integrated circuits. A hybrid integrated circuit is one in which a semiconductor chip, usually a silicon chip, is connected to a bonding pad region of a circuit pattern on a ceramic substrate. The encapsulant of the Wong patent is sufficiently fluid to be screen printable and it adheres well to gold and tantalum nitride surfaces, which are sometimes the metals used as the circuit pattern. One problem with that particular resin is its tendency to crack at high temperatures; the uncured molecule contains OH terminations which create a water by-product during cure which in turn makes it susceptible to cracking. Another problem is the relatively long curing time at an elevated temperature that is required.
The U.S. application of C. P. Wong, Ser. No. 229,403, filed Aug. 8, 1988 U.S. Pat. No. 4,888,226, describes the use of a silicone resin gel as an encapsulant, particularly for wire-bonded integrated circuits. The disclosed resin comprises polydimethylsiloxane and/or polymethylphenylsiloxane having vinyl and hydride functional components, and tests described in the patent application show that it is particularly resistant to high-temperature cycling. A more detailed discussion of silicone gel encapsulants is included in the paper "High-Performance Silicone Gel as Integrated-Circuit-Device Chip Protection," by C. P. Wong, included in the book "Polymeric Materials for Electronics Packaging and Interconnection," J. H. Lupinski and Robert S. Moore, editors, American Chemical Society, Washington, D.C., 1989. A discussion of how silicone resin encapsulants compare with other encapsulants such as epoxies, polyimides and polyurethanes is contained in the paper "An Overview of Integrated Circuit Device Encapsulants," by C. P. Wong, Journal of Electronic Packaging, June, 1989, Vol. 111, pages 97-107. One problem with the material described in the Wong patent application is that it is not screen printable; that is, in its uncured state it is too thick to be deposited on a hybrid integrated circuit by conventional screen printing methods.
There is therefore a continuing need in the industry for a hybrid integrated circuit encapsulant that is screen-printable, has a reasonably short cure time, is not susceptible to high temperature cracking, but which obtains the benefits of known silicone encapsulants such as good adhesion to ceramics and metals, resistance to atmospheric contaminants, good structural characteristics and good temperature-humidity-electrical bias (THB) characteristics.